unless you have 108 bucks for a herpstat you wont get a proportional, and, you dont NEED a proportional t-stat.
A rheostat is something I would never use, if your room temp goes up 7 degrees, so does your heat pad.

I have 3 reptitemps in use for over 2 years, no problems.
I also have a johnson controls a419 that is ok, but the herpstat is a joy, I have one on my incubator.